Text: The bony labyrinth may be briefly described as a chamber, the "Vestibule" which sends one prolongation forward (the "cochlea"), three others backwards (semi-circular canals) and has its outer and inner walls perforated, the outer by the fenestra ovalis, in which lies the base of the stapes, and by a round hole closed by membrane, and called the fenestra rotunda; the inner by a series of holes in a depression called the "Fovea hemispherica", which transmit branches of the auditory nerve from the internal auditory meatus in which lie the auditory and facial nerves. By these two lateral perforations it communicates with the cavity of the tympanum externally, and with that of the cranium internally. Close behind the "Fovea hemispherica" is a small canal, the "Aquaductus Vestibuli". The bony semi-circular canals are three tubes bent so as to form about two-thirds of a circle. They are situated at the upper and back part of the vestibule with which they communicate by five openings, one end of the superior having an opening common also to the posterior semi-circular canal. Each tube at one end has an expansion called an "Ampulla". These canals are called from their position, superior, posterior and external. The superior is vertical and transverse, the posterior is vertical and longitudinal, and the external is horizontal. The directions of these canals, or the planes in which they lie, will be best understood by placing a book with the two covers at right angles to one another, upright on end on a table, so that one of the covers faces the reader, the other being at right angles to the side of the table at which he is seated. Then the reader will be on the outer or tympanic side, the side of the cranial cavity. The plane of the table will represent the plane of the external or horizontal canal, the plane of the cover opposite to the reader the posterior, and that at right angles to the side of the table at which he is seated the superior canal, which is also the most anteriorly placed of the three. Thus it will be seen that the planes of these three canals are the three planes of a cube. [With regard to the terms anterior, posterior, external, and internal, it may be necessary to explain that anterior means on the side towards the face; posterior on the side towards the back of the head, and external and internal remote from or near to an antero-posterior axis drawn from the face to the back of the head.]

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